1. Field of the Invention
This invention relates generally to the manufacture of ceramic items and particularly to improvements in the manufacture of thin flat ceramic substrates by the tape casting process.
2. Description of the Prior Art
The art of manufacturing thin flat ceramic items has been developed to meet the need for capacitor dielectrics and for substrates in microelectronic and semiconductor circuitry, among other uses. The common method for manufacturing such articles is by the tape casting process. In this process a fluid ceramic slurry, called a slip, is made by mixing together a finely ground ceramic powder, such as aluminum oxide (alumina), with suitable amounts of an organic binder, a volatile solvent, usually a plasticizer, and possibly small amounts of other materials, depending on the product requirements. Typical ingredients are identified, and the mixing and casting process are fully described in U.S. Pat. No. 2,966,719, issued to J. L. Park, Jr. on Jan. 3, 1961 as assignor to American Lava Corporation, and in U.S. Pat. No. 3,698,923, issued to H. W. Stetson et al. on Oct. 17, 1972 as assignors to Western Electric Company, Incorporated. The reader is referred to these patents and also to the earlier U.S. Pat. No. 2,582,993, issued to G. N. Howatt on Jan. 22, 1952, for details of the tape casting process and its developmental history as well as for the characteristics of the resulting ceramic product.
Briefly, the process involves discharging the above-described ceramic slip, which has a viscosity and consistency approximately the same as heavy cream, from a reservoir onto a supported, moving surface, preferably a plastic tape or film such as cellulose acetate, polytetrafluoroethylene ("Teflon"), or glycol terephthalic acid ester ("Mylar"). The film is usually in the form of an elongated strip several hundred feet long and from one half to two feet wide, wound on a storage reel mounted next to the reservoir.
The tape is led from the storage reel under the reservoir to a takeup reel, and a suitable drive mechanism moves the tape in a substantially horizontal path from the storage reel to the takeup reel. The cast slip is distributed evenly on the moving tape by an inverted dam forming the outlet of the reservoir or by a doctor blade in order to form a layer of uniform and controlled thickness.
As the layer of cast slip is conveyed on the plastic tape from the reservoir, the volatile solvents evaporate, the process of driving off the solvents being accelerated by passing the tape through an elongated, heated drying chamber. The resulting product is a ceramic tape that is aptly described as "leather hard". This tape can be punched or sliced into the shape and size desired for the substrate or other item and then be fired at high temperature (e.g. 1500.degree. C) to produce a rigid ceramic article.
In many applications the thickness, and particularly the uniformity of thickness, of the resulting substrates must fall within very narrow tolerances. For example, the "green" tape (i.e. the leather-hard product before firing) may be 17-18 inches wide yet only 0.03 inches -0.04 inches thick and have a tolerance on thickness variation of only a few thousandths of an inch. Although the doctor blade is accurately machined and is adjustable in height over the surface of the base strip, the combined effects of irregularities in the shape of the doctor blade, gravity and surface tension of the cast slip tend to produce a variation in thickness across the width of the slip amounting to 0.003-0.007 inch or more.
Assuming best possible adjustment of the doctor blade, the effect of gravity, surface tension and shrinkage usually creates a saddle-shaped cross section in which the tape is thicker than desired near the edges and slightly thinner in the center portion. To meet the most rigid thickness variation tolerances, it is often possible to use only the center portion of the "green" tape for making substrates, which results in waste and increased cost of manufacture.